The gamma camera is substantially constituted by three fundamental elements: a collimator a scintillation structure and one or more photomultipliers.
The collimator is placed between an object that emits gamma radiation and the scintillation structure and has the function of allowing the passage only of the radiation directed perpendicularly to the scintillation structure screening all radiation directed perpendicularly to the scintillation structure shielding all radiation directed in different directions.
It should be noted that gamma radiation, composed of gamma photons, cannot be deviated by optical lenses (e.g. it takes place for the photons emitted in the visible range in traditional photography) because of their penetrating power and they also do not have electrical charge. The photon beam is then modulated by means of the collimator which acts shielding most of the emitted photons. The scintillation structure is constituted by a single planar crystal or by a plurality of crystals able to receive gamma photons and to transform them into photons in the visible range (light photons).
The photomultiplier is connected to the scintillation structure at the opposite side from that of the collimator through an appropriate optical connection and its function is to detect the light photons transforming their energy into an electrical signal that is amplified and carried towards the processing circuits to recreate the image of the radiation source.
The gamma camera is used in “imaging” systems for diverse applications, such as diagnostic applications (like PET, SPECT and conventional scintigraphies), in Astrophysics and in systems for industrial non-destructive tests. The intrinsic spatial resolution of the gamma camera depends, among other matters, on the dimensions of the crystals that compose the scintillation structure.
To improve the intrinsic spatial resolution of the gamma camera, scintillation structures have been developed which comprise a plurality of individual scintillation crystals with dimensions in the order of one millimeter, flanking each other (crystal matrices).
In a known method to obtain a scintillation structure that allows a high spatial resolution, so-called matrix structures are created, i.e. structures in which individual crystals in the form of rods are locked by means of epoxy resins whose function is to keep the crystals mutually linked and equidistant.
Consequently, said matrix structures are directly usable as scintillation structures.
As stated, the purpose of the scintillation structure is to convert the energy of the incident gamma photon into light photons.
In particular, for each gamma interaction a certain number of photons are emitted in the visible range, which number depends, as a first approximation, on the energy released by the incident gamma photon. Depending on the type of interaction, the energy release can be complete (photoelectric effect), or partial (Compton effect).
In the ideal case, the incident photon releases all its energy to the crystal and a perfect proportionality is maintained between the energy of the gamma photon and the intensity of the light that reaches the photomultiplier.
In reality, the gamma photon can interact with an electron of the crystal through the Compton effect undergoing a deviation from the original direction and depositing only a part of its energy. This process can be repeated, originating successive deviations. The diffused photon, though with less energy than the initial one, can traverse the dividing layer of epoxy resin and produce a new scintillation in neighboring crystals.
The same gamma photon can thus produce multiple scintillation points in neighboring crystals. In general, this entails an erroneous calculation of the position of the interaction of the gamma photon.
The individual interaction event between photon and crystal is seen by the electric device reading the collected signal as the sum of the energy contributions released into the crystals in the individual interactions. In general, the utilization electronics are based on the method of the charge barycenter recorded on the entire surface area of the crystal and consequently the final value of the recorded position includes all the interaction effects that occur in the crystal.
The probability that a Compton effect may take place in a crystal is in any case linked to the energy possessed by the primary photon that interacts with the scintillating material. For SPECT applications, this probability is low if referred to the low energies of the radio tracers used. As energy increases, instead, the probability becomes progressively higher, until reaching very high values for applications with 511 keV photons in PET applications.
A possible solution to the problem described above is provided in the document U.S. Pat. No. 6,734,430 which discloses a method for obtaining a collimator made of metallic material having a high atomic number that integrates the scintillation structure. In this, it is possible to achieve the attenuation of the gamma photons that may pass from one crystal element to the other, obtaining better results than those achieved with separating epoxy resins. Metals having high atomic number (and hence high density) are more difficult to traverse for gamma photons than epoxy resins.
In the collimator described in the aforementioned prior art document, the individual scintillation crystals are integrated in the channels of the collimator, in particular each individual crystal is separated from the other crystals of the matrix and inserted into the holes present at the base of the collimator.
However, because of obvious construction difficulties, the solution with a single collimation structure and integrated crystals does not allow an optimal machining of hygroscopic crystals which need to be completely isolated from the surrounding environment.
Moreover, the insertion of individual crystals into the collimation channels entails additional drawbacks linked to the need to anchor the individual crystals to prevent them from moving along the channels of the collimator.
Additionally, positioning each crystal in such a way that it is perfectly aligned to the channel in which it lies and that all the crystals are arranged with the respective walls oriented towards the photomultiplier to form a single plane, perfectly orthogonal to the channel of the collimator, is very difficult and it requires considerable resources.